The development of atomic clocks with high performance, compact structure, and low power consumption is of great importance to many scientific research and industrial production. Applications of atomic clocks comprise gravitational wave detection, verification of general relativity, new-generation satellite navigation and positioning, network synchronization, and time keeping systems on mobile platforms. In many types of atomic clocks, the alkali-metal vapor cell atomic clock has attracted more and more attention because of its simple structure and high stability. In recent years, with the development of technologies such as laser devices, the laboratory-use alkali-metal vapor cell atomic clocks based on technologies such as optical-microwave double resonance (OMDR), coherence population trapping (CPT), pulsed optical pumping (POP) and pulsed integrating sphere, have achieved the short-term stability of a level of 1-4×10−13τ−1/2. Comparing with the present commercial lamp-pumped rubidium atomic clock, the laboratory-use alkali-metal vapor cell atomic clocks have improved the short-term stability by about two orders, and the short-term stability keeps approaching that of a hydrogen maser.
The main factors limiting the short-term stability of the above-mentioned alkali-metal vapor cell atomic clocks include the quantum projection noise, the relative intensity noise of the laser, frequency modulation to amplitude modulation conversion noise, and the Dick effect due to the noise of the crystal oscillator. As early as 1990, G. J. Dick discovered the Dick effect. However, as a magnitude of the Dick effect is small, it was not the main restriction factor for the short-term stability of the atomic clock at that time. However, along with the application of the pulsed Ramsey coherence population trapping (Ramsey-CPT) atomic clock, the short-term stability of the atomic clock gradually approaches the limit of the quantum projection noise, and the Dick effect has then gradually become an important factor restricting the short-term stability of the alkali-metal vapor cell atomic clock.
In order to reduce the Dick effect of the atomic clock, an oscillator with better phase noise properties (such as a cryogenic sapphire oscillator) can be used instead of the ordinary crystal oscillator. However, the above-mentioned oscillator with better phase noise properties is complicated in structure and expensive, which will cause the alkali-metal vapor cell atomic clock with the above oscillator to have a large size and high costs.